WASHINGTON -- Nearly every day over the past six months there has been some new development in the White House sex and perjury scandal: a legal battle, a grand jury witness, a statement on the courthouse steps.

But for all the news -- setbacks, victories, revelations -- the basic story strayed little from the one the public heard when the name Monica Lewinsky first surfaced in mid-January.

That's all changing now.

In the past week, independent counsel Kenneth W. Starr secured what he needed most, what has been most elusive to him and what should finally bring his investigation -- four years old this week -- to some sort of conclusion.

Starr received the promise of testimony from the two central players in the saga: Lewinsky, who agreed to cooperate in exchange for complete immunity from prosecution for herself and her mother, and President Clinton, who volunteered to testify only after receiving an unprecedented presidential subpoena that has been withdrawn.

Through these deals, the first hints of the upcoming showdown between the president and the prosecutor are taking shape. And the prospect that the public will get closer to the truth after six months of rumor, gossip and endless TV talk is becoming more likely.

The developments of the past week, including an appeals court ruling that Clinton confidant Bruce R. Lindsey cannot invoke attorney-client privilege to avoid testifying before Starr's grand jury, and news that Lewinsky turned over to Starr a dress that may contain forensic evidence of a relationship with Clinton, "drastically affects the landscape of the story," says Benjamin Insberg, a former general counsel of the Republican National Committee.

"There's been a sea change in this story," said Ginsberg. "And the evidence of that is that the president realized he had to testify."

Starr has been trying to determine whether Clinton and Lewinsky had a sexual relationship -- which both initially denied under oath -- and whether the president urged the former White House intern to lie about it or otherwise obstructed justice.

As of late last week, White House officials were saying that the president planned to stick to his story that he had no sexual relationship with Lewinsky. But they admitted that, unlike most presidential decisions, this one was being threshed out by Clinton and a small cadre of lawyers only -- White House counsel Charles Ruff and private outside attorneys David E. Kendall, Robert S. Bennett and Mickey Kantor -- and that few others were privy to their thinking.

Clinton denied a relationship with Lewinsky in his sworn deposition in the Paula Jones sexual misconduct case. And days after the Lewinsky scandal broke in January, he looked the nation straight in the eye at a televised White House event, wagged his finger and emphatically declared that he did not have sexual relations with "that woman, Miss Lewinsky."

To say anything different now would be an admission of perjury in the Jones case -- a charge that may or may not be considered serious enough to warrant impeachment hearings.

Significance of the dress

Lewinsky is expected to testify that Clinton did, in fact, swear falsely when he denied a sexual relationship with her. She also is expected to say that she and Clinton had some sort of understanding that their relationship was to be kept secret.

But even if her testimony is corroborated by the 20 hours of phone conversations secretly recorded by Linda R. Tripp, Lewinsky's word is likely to be no more weighty than Clinton's.

The emergence last week of Lewinsky's long-rumored dress -- the most startling, sensational and perhaps important development of the week -- changes the equation.

"The dress takes it out of the 'he-said, she-said' stage," said Ginsberg.

But the dress only offers relevant and conclusive physical evidence if the FBI lab, which received the dress from Starr late last week, determines that there is DNA evidence on the dress that can be traced to another person.

If genetic material is found, prosecutors would then have to obtain a biological sample from Clinton -- perhaps from a drop of blood, a hair or a dab of saliva -- in order to link the evidence to him. Targets of criminal investigations cannot legally resist providing such physical evidence.

Other evidence disclosed last week, messages to Lewinsky from Clinton on her home answering machine, has cast some doubt on Clinton's story. The messages are not said to be romantic or sexual, but the fact that the President of the United States was calling an intern at her home suggests some sort of intimate relationship.

Still, short of possible DNA evidence proving the contrary, some legal experts say Clinton is wise to continue denying a sexual relationship with Lewinsky.